For a development process in a recent development apparatus in a nonmagnetic single-component development system, a contact development system is known that uniformly presses a development roll with toner (developer) adhering thereto against a photoreceptor for development (see FIG. 3). In FIG. 3, reference numerals 51, 52, 53, 54, and 55 denote a development roll, a regulation blade, a supply roll, a photoreceptor, and toner, respectively.
In general, while an electrostatic latent image means forms an electrostatic latent image on a surface of a photoreceptor drum based on predetermined image information, a toner supply roll supplies toner onto a surface of a development roll, and the toner supplied onto the development roll is regulated to a uniform thickness by a developer amount regulation member (hereinafter also referred to as a “regulation blade”) to form a uniform thin layer of toner on the surface of the development roll.
The development roll having the toner thin layer formed on the surface causes the toner to sequentially adhere to the electrostatic latent image formed on the photoreceptor drum in a nip portion or a proximity portion to the photoreceptor drum. The toner development is performed in this way.
To acquire a uniform high-density image in such a contact development system, it is important to form a uniform and thin toner adhesion layer on the development roll surface. For this purpose, a sufficient toner conveyance amount must be maintained by forming a uniformly roughened surface at the development roll surface and a uniform gap must be ensured from the regulation blade.
The development roll typically has a structure formed by accumulating a conductive rubber-like elastic body on a circular cylindrical surface of a core material made of metal, a fibrous material, or a resin molded body. A coating layer made of a resin is disposed on the surface of the rubber-like elastic body to take measures against (1) a problem that a component contained in rubber migrates to a surface to contaminate an opposite material, (2) a problem that a volume resistance value largely varies due to environmental changes, (3) a problem that a large coefficient of dynamic friction necessitates a large torque for rotation because of a relation of friction with a contacting opposite material, and (4) problems of tack of toner, tack to an opposite material, occurrence of frictional noise, dirt adhesion, etc. due to a tack property and a high friction coefficient property.
However, since a hard component is often applied as the coating layer, characteristics of rubber may impaired or deformation of rubber may result in a crack in the coating layer or abrasion of the coating layer.
Although methods of achieving a soft coating layer with low tackiness and low friction include a method of adding an oil component, an oil component is less persistent in terms of low tackiness and low friction and may bleed on a surface and cause contamination of opposite and peripheral portions.
Patent Document 1 discloses that a coating layer made of a reaction product (fluorine-containing polyurethane) between a urethane resin and a fluorine-containing compound having a functional group is formed on a rubber-based conductive layer surface.
This coating layer may block a migrating contaminant as described above in the rubber-based conductive layer and therefore may prevent contamination of a photoreceptor drum surface due to the migrating contaminant.
However, even when a development roll includes the coating layer, toner is rubbed between the development roll surface and a thinning blade or a photoreceptor drum, and therefore, frictional heat due to rubbing facilitates the toner to melt.
If the toner melts, a so-called filming phenomenon of causing the toner to fixedly adhere to the development roll surface appears and adversely affects a print image. The filming also appears on the photoreceptor drum surface and, in this case, a filming portion appears on an image as a streak-like pass-through. The deterioration of toner facilitates melting and causes this problem to easily occur.
Particularly, because of increases in speed and resolution of recent image forming apparatuses, toner has a lower melting point and a smaller diameter and therefore more easily melts due to heat.
The filming is notable in a toner sealing portion applying a strong stress to toner and, in this case, the sealing properties of toner are affected, resulting in a problem such as a toner leak in the worst case.
Patent Document 2 discloses a technique of forming a coating layer from a resin containing a reaction product of a fluorine-containing resin having a functional group reactive with an isocyanate group, a polyol compound, and a polyisocyanate compound.
While maintaining characteristics of fluororesin such as prevention of contamination of an opposite member such as a photoreceptor, prevention of adhesion of toner, control of charging of toner, resistance adjustment, and control of friction coefficient, this technique enables acquisition of favorable flexibility and can achieve a longer life from favorable conformability to elastic deformation of rubber, foam material, etc. without causing a trouble such as breaking. This consequently eliminates exudation of a rubber compounding agent from a crack of the coating layer, adhesion of toner to the crack, a change in friction coefficient, etc. leading to inability to achieve an initial purpose in an early stage, and an OA device member excellent in long term durability is acquired.
However, increases in speed and resolution of recent image forming apparatuses and a lower melting point of toner have given rise to a problem of inability to sufficiently deal with a trouble.
Moreover, since a rubber singly-layer roll has a large coefficient of dynamic friction on a surface and necessitates a large torque for rotating a development roll because of a relation of friction with contacting members such as a toner thinning blade and a toner supply roll, a more powerful motor is required, which is an obstacle for reductions in size, power, and cost of an apparatus.
On the other hand, particles are dispersed in a surface of a coating layer to form uniform roughness on the surface to maintain a toner conveyance amount in a recently generally used technique.
By dispersing particles in the surface of the coating layer to form a convex shape, point contact of toner can be achieved to prevent the deterioration of toner.
Patent Document 3 discloses a coating layer having a material with low compression set, for example, a silicone resin and a urethane resin uniformly dispersed in a binder as resin particles for the purpose of surface roughening.
However, if hard resin particles etc. are used as the particles, since a large amount of particles must be added for forming concave and convex uniformly at the development roll surface, the resin particles occupy a large portion of the development roll surface and thereby make the coating layer itself hard.
It is confirmed that the hardness of the coating layer itself and the hardness of convex portions (particles) affect a toner stress.
Since the hard resin particles form the convex portions of the coating layer, when particles come into contact with toner, the toner tends to deteriorate, and silica etc. of an external additive component of the toner adhere to the wall surfaces of the particles and act as a factor of the filming.
Patent Document 4 discloses a technique of increasing a size of particles used in a surface layer so as to increase roughness of a roll surface because a toner conveyance amount can be made larger in accordance with a magnitude of roughness of the development roll surface.
However, excessively large surface roughness leads to the coarseness and unevenness of an acquired image and the degrading in fogging characteristics and makes it difficult to form uniform roughness across the entire roll surface.
Variations in roughness of the roll surface may result in a biased toner conveyance amount or an uneven toner charge amount.
On the other hand, for a polyurethane coating layer covering a conductive rubber layer, a technique is disclosed for using a reaction mixture containing polyol, an isocyanate compound, and a both-end reactive silicone oil to introduce a siloxane component inside a polyurethane molecular chain rather than simply binding the siloxane component as a side chain to the polyurethane molecular chain so that a siloxane molecule is introduced in a manner crosslinking the polyurethane molecules. This technique enables provision of a development roll excellent in filming resistance while retaining advantages of conventional polyurethane layers capable of solving a problem of contamination of the photoreceptor drum surface and a problem of drive torque of the development roll (Patent Documents 5, 6, and 7).
By subjecting a volatile silicone oil to a reaction condition of polyol and the isocyanate compound, since the silicone oil is less compatible with the isocyanate compound, when the mixture thereof is applied to a rubber surface and fired to dry a solvent, the volatile silicone oil is no longer able to dissolve in the mixture, forms a multiplicity of minute droplets before subsequent volatilization, and therefore forms a film having holes (cissing, dents) generated at corresponding locations. By forming the coating layer as a porous body in this way, the coating layer surface is configured as a roughened surface.
However, if the mixture is applied to a rubber substrate to the extent of 10 μm, actually generated dents have a width exceeding 20 μm and a depth of about 3 μm, therefore make the conveyance amount too high, causing a problem of higher cost, and allow toner to enter and accumulate in dent portions, causing problems of deteriorating the toner, acting as a factor of the filming, etc.